Retaining Walls Guide: How to Choose the Right Walls

A retaining wall holds back, or “retains”, the soil or water behind it. The retained material can be natural soil, earth fill, sand or a liquid, and the pressure it puts on the wall can be very large. A good-looking retaining wall idea is not enough on its own: the wall has to be designed by someone who can calculate that pressure and size the wall to resist it.

The design also has to deal with water. A wall that cannot drain will carry extra water pressure after heavy rain or when the water table rises, so weep holes, drainage fill or a drainage layer behind the wall are part of the design. Civil engineers use retaining walls for construction on or along slopes and on sites where there is no room to cut the ground back to a safe slope.

A retaining wall is built to resist the lateral pressure of soil or water wherever the change in ground level is steeper than the soil’s angle of repose. Walls are built from many materials to hold soil that would not stay on that slope by itself. They are also used to hold soil at two different levels where the landscape has to be reshaped for a specific purpose, such as terraced hillside farming or the approaches to an overpass.

Retaining Wall Design Consideration

Retaining walls are vertical or near-vertical structures that hold material on one side and stop it collapsing, slipping or eroding. They support ground that would otherwise slump once the angle of repose of the soil is exceeded.

The main types of retaining wall failure are:

Toppling (overturning): the overturning moment from the lateral earth pressure exceeds the resisting moment from the weight of the wall, and the wall rotates forward about its toe.

Bearing capacity failure: the soil below the wall fails because the load on it exceeds its bearing capacity.

Sliding: the wall slides forward because friction and passive resistance at its base are not enough to resist the lateral push.

Internal failure: the material the wall is built from, or its structural frame, fails.

The starting point of any design is the tendency of the retained material to move down the slope under gravity. That tendency produces lateral pressure on the back of the wall, which depends on the angle of internal friction (φ) and cohesion (c) of the retained soil, and on how much, and in which direction, the wall moves. In a homogeneous cohesionless soil the lateral pressure is zero at the top of the wall and increases linearly with depth, which is why walls are thickest at the base.

Retaining Wall Categories

Earth retaining structures fall into a few main categories, each with sub-types:

Gravity Retaining Walls

Gravity retaining walls rely on their own weight to hold back the material behind them. Because weight is the only thing resisting the lateral earth pressure, they are built massive to prevent sliding, overturning and bearing failure. Stone masonry, mass concrete and gabion walls all work this way.

Reinforced Retaining Walls

Reinforced concrete and reinforced masonry walls built on spread foundations are designed with the same stability checks as gravity walls. In a cantilever wall, the steel bars give the stem and base slab the bending strength to act as a single L- or T-shaped unit, and the weight of soil on the heel slab, together with the wall’s own weight, resists overturning. This lets the wall be much thinner than a gravity wall of the same height.

Reinforced soil walls take a different route: layers of geogrid are laid in the compacted backfill and tied to a facing, so the reinforced block of soil itself becomes the gravity mass.

Anchored Retaining Walls

An anchored retaining wall is tied back with cables or tendons anchored in the rock or soil behind it. The anchors carry part of the lateral load, so the wall itself needs less mass.

The anchors are drilled into the ground behind the wall and expanded at the far end, either mechanically or by injecting grout under pressure, which forms a bulb in the soil. The method is more complex to build, and it suits sites where the loads are very high or where the wall has to be slender.

If a reinforced soil wall is on your shortlist, our guide to designing a geosynthetic reinforced retaining wall explains the parts of the wall, how the reinforced block is sized and the facing options available.

Retaining Wall Blocks

Retaining walls are also used in landscaping, around house gardens, office grounds and outdoor seating areas, usually built from retaining wall blocks. The same blocks are used for structural walls around houses, offices and industrial plots. Concrete blocks are the most common because they are easy to get and simple to lay.

Blocks give a landscaping job both a finished look and a working structure. Besides concrete blocks, common retaining wall materials include stone veneer, poured concrete, brick, timber, boulders, gabions, natural stone and limestone.